1. Field of the Invention
The present invention relates to a semiconductor device and particularly relates to a semiconductor device having a heterojunction bipolar transistor.
2. Description of the Related Art
Transistors used for a semiconductor device are roughly divided into bipolar transistors and field effect transistors, such as MOS (metal—an oxide film—a semiconductor) field effect transistors.
A heterojunction bipolar transistor (hereinafter, also referred to as an HBT) as one of the bipolar transistors is a bipolar transistor wherein a material having a wider band gap than that of a base layer is used for an emitter layer, by which the injection efficiency (emitter efficiency) of electrons from the emitter layer to the base layer can be kept high even when the impurity density of the base layer is made higher than that of the emitter layer.
Accordingly, even when a base layer thickness is reduced, resistance of the base layer can be maintained low and punch-through of the base layer is suppressed so as to maintain breakdown strength between the emitter and the collector high. Basically, it is a device having the excellent properties of high speed and high breakdown strength.
Also, since the HBT has a high current drive ability, it is excellent as a current amplifier (power amplifier: hereinafter, it is also referred to as a PA) device. Furthermore, due to the advantage of an easy, single, power source operation, it has come to be widely used as a mobile communication terminal PA in recent years.
The Power-Added Efficiency (PAE) is known as an index indicating the efficiency of a PA. The PAE is defined as the ratio of an added power, that is the difference between an output power Pout and an input power Pin, to a direct-current power application Pdc. The larger the value, the more the power consumption of a power amplifier can be suppressed, so that the value is used as one of the important indexes in a power amplifier. It is particularly important in a mobile communication terminal because the power consumption by a transmission side power amplifier occupies a large ratio in the whole power consumption.
In the above HBT, as an HBT being lattice matched with the InP, for example, the configuration of successively stacking on a substrate a collector layer made by n-type InGaAs, a base layer made by p-type InGaAs and an emitter layer made by n-type InP (also called an HBT of the first generation) is known.
The HBT of the first generation is capable of being driven at a very high speed but has a defect that the breakdown strength between a base and a collector is weak due to a narrow band gap of the collector layer.
On the other hand, the configuration of a double hetero structure of, for example, successively stacking on a substrate a collector layer made by n-type InP, a base layer made by p-type InGaAs and an emitter layer made by n-type InP (also called an HBT of the second generation) is known.
The HBT of the second generation has the configuration that the collector layer has a wide gap compared with the case of the HBT of the first generation, so that the breakdown strength can be improved.
However, since the electron affinity of the collector layer is smaller than that of the base layer, a current flow from the base layer to the collector layer is hindered, so that it is necessary to make the energy band profile between the base and the collector graded, etc. for improvement.
In the case of configuring a power amplifier by using an HBT as the above, one of demands to the device side to improve the PAE is a reduction of a knee voltage Vk in the Ic-Vce characteristic.
To reduce the knee voltage, an offset voltage Voffset as a rising voltage of the Ic in the Ic-Vce characteristic has to be made small.
The above offset voltage Voffset is almost determined by the difference of a rising voltage Vteb in the forward direction between the emitter and the base and a rising voltage Vtbc in the forward direction between the base and the collector, that is Vteb-Vtbc.
In a heterojunction system wherein the electron affinity of the emitter is smaller than that of the base, however, in the case where there is no energy discontinuity of a conduction band end between the base layer and the collector layer, when assuming that the energy discontinuity amount of the conduction band end generated between the emitter layer and the base layer is ΔEc, a value of Vteb-Vtbc becomes approximately ΔEc.
Namely, in the above HBT of the first generation, the value of Vteb-Vtbc becomes large.
On the other hand, since the above HBT of the second generation has the configuration wherein the emitter layer and the collector layer become symmetrical about the base layer, the value of Vteb-Vtbc can become close to almost 0, but when an interlayer of a graded energy band profile is provided between the base and collector, the value of Vteb-Vtbc becomes large.
On the other hand, in an HBT of a so-called staggered type heterojunction system wherein the emitter and collector have a larger electron affinity than that of the base, the value of Vteb-Vtbc becomes almost 0, so that it is advantageous for reducing the offset voltage Voffset.
FIG. 1 is a sectional view of an example of the above staggered type HBT.
In the above semiconductor device, for example, a sub-collector layer 2 made by n+-type InGaAs, a collector layer 3 made by n−-type InP, a base layer 4 made by p+-type GaAsSb, an emitter layer 5 made by n-type InP, and a cap layer 6 made by n-type InGaAs are successively stacked on a substrate 1 made by a semi-insulative single-crystal InP doped with Fe.
An emitter electrode 7 is formed on the cap layer 6. Also, a mesa structure is formed for forming an ohmic contact to a base and a collector, and a part of the base layer 4 contacts a base electrode 8 and a part of the sub-collector layer 2a contacts a collector electrode 9, respectively.
These electrodes are formed, for example, by a stacked body of Ti/Pt/Au.
Also, the surface of the semiconductor not contacting the electrodes is covered with an insulation film 10 made, for example, by Si3N4.
FIG. 2 is a schematic view of an energy band structure corresponding to the HBT shown in FIG. 1 and shows energy levels of a conductive band C and a valance band V.
The figure shows the energy band profile wherein electron affinities of the emitter and the collector are larger than an electron affinity of the base.
In such an HBT, as explained above, there is an advantage that the value of the offset voltage Voffset can be easily made close to 0.
For example, The Japanese Unexamined Patent Publication No. 5-41388 and The Japanese Unexamined Patent Publication No. 6-326120 describe the above HBT of the first generation, and The Japanese Unexamined Patent Publication No. 2002-270616 describes the above staggered type HBT.
Also, the patent articles The Japanese Unexamined Patent Publication No. 6-260493, The Japanese Unexamined Patent Publication No. 8-250509, The Japanese Unexamined Patent Publication No. 11-176838, and The Japanese Unexamined Patent Publication No. 2002-2708174 describe other kinds of HBTs, such as an AlGaAs/GaAs based HBT.
However, in the above staggered type HBT, when an energy discontinuity amount ΔEc of the conduction band end generated between the emitter layer and the base layer is too large, there arises a problem that emitter resistance becomes high and the electron injection efficiency from the emitter layer to the base layer declines.
Also, when the ΔEc is too large, a problem is liable to arise that the breakdown strength between the base layer and the collector layer declines.
Furthermore, there is another disadvantage that, in an HBT having the configuration shown in FIG. 1, a part of the surface of the pn-junction between the emitter and the base exposes in a production process, which leads to an introduction of a defect in some cases, so that reliability declines.
As explained above, a staggered type HBT has the advantages that the value of the offset voltage Voffset is easily made to be 0 and the knee voltage can be reduced, while basic characteristics of a transistor are deteriorated, such as an increase of emitter resistance, a decline of breakdown strength between the base layer and the collector layer or a decline of reliability due to a defect introduction, etc. in some cases.